The present invention relates to a method for treatment of sewage sludge and in particular to a method for reducing the viscosity of dewatered or concentrated sewage sludges having a solids concentration of at least 10% (w/w).
Sewage sludges typically have concentrations of 0.5 -3.0% (w/w) solids. The sludges may be thickened, with removal of water, by a variety of methods, including gravitational thickening, flotation thickening, centrifugation and screw and belt pressing. Surfactants may be added to sludge to facilitate particle flocculation as part of the steps to thicken or concentrate the sludge.
Sludges of about 3-5% (w/w) solids are sufficiently low in viscosity that they are still capable of being pumped, which facilitates transfer of the liquid sludges to and from storage and transporting tanks. In addition, the ability to pump sludges facilitates transfer of the sludges to land application equipment and also application of the liquid sludges to the land by surface spraying or subsurface injection.
There is substantial interest in concentrating liquid sludges, having 0.5-5% (w/w) solids, to reduce the volume of material required for transport, storage and/or further handling. However, the high viscosity of more concentrated sludges makes them less suitable for pumping, spraying and injection. It is also recognized that polymeric and other kinds of flocculating agents and conditioners, used to produce concentrated sludges, may also contribute to the increased viscosity of these sludges. Hence, there is a great interest in finding cost effective methods for reducing the viscosity of more concentrated sludges so that they are more easily pumped and/or sprayed and/or injected. and/or further processed.
U.S. Pat. No. 5,188,741 describes a process to treat sewage sludge to increase sludge solids content and to decrease viscosity, which includes a step of heating to about 500xc2x0 F. (about 260xc2x0 C.) at a pressure of 750 psig under a blanket of nitrogen. U.S. Pat. No. 5,783,073 describes a process and apparatus to liquefy sludges containing up to 10% solids at a pH of greater than 12. U.S. Pat. No. 5,618,442 describes a process for destroying harmful pathogens in sludge under alkaline conditions in a pasteurization chamber. U.S. Pat. No. 4,471,916 describes a process for reducing sludge viscosity involving mechanical treatment and addition of an alkali metal silicate. U.S. Pat. No. 5,356,540 describes a pumpable aqueous sewage composition that contains a non-ionic water-soluble alkoxylated alkyl phenol, and a process for preparing aqueous sewage sludge having a solids content of 30-65% by weight. U.S. Pat. No. 5,681,481 describes treatment of sewage sludge with a solids content of less than 10%, at a pH of 12.0 or higher for a period of time e.g. at least 50xc2x0 C. for at least about 12.0 hours.
Methods described to date for producing low viscosity sewage sludge concentrates are either limited to the sludges having  less than 10% solids content or are disadvantageous in that extreme conditions of temperature and/or pressure are required to reduce viscosity to facilitate pumping and/or land spraying and/or injection of the sludges.
An improved method has been developed for production of concentrated sludges having low viscosities.
Accordingly, one aspect of the present invention provides a method for reducing sludge viscosity of a sewage sludge having a solids concentration of at least 10% (w/w), comprising the steps of:
(a) increasing the pH of the sludge to the range of 9.5-12.5;
(b) selecting at least one step from
(i) maintaining the sludge at the pH of (a) and at a temperature of 10-37xc2x0 C. for a period of at least one day, and (ii) adding one or more inorganic or organic chemicals to the sludge, such chemicals contributing to viscosity reduction;
(c) incubating the sludge by maintaining the resultant sludge at a temperature in the range of 40-100xc2x0 C. for a period of time of at least one hour;
(d) subjecting the sludge to physical shearing or disintegration; and
(e) subsequently discharging the sludge.
In embodiments of the process of the present invention, step (b)(i) is selected, step (b)(ii) is selected or both step (b)(i) and step (b)(ii) are selected.
In the embodiment in which step (ii) is selected, at least one of a sodium or potassium salt is added, especially at least one of sodium or potassium chloride, optionally with addition of an oxidizing agent is added in step (c) e.g. sodium hypochlorite or hydrogen peroxide.
In another aspect of the present invention, a low viscosity sewage sludge is obtained, which may be further processed by physical, chemical and/or biological methods, may be applied to land by spraying, injection or other methods, or disposed by any method.
A further aspect of the present invention provides an apparatus for reducing the sludge viscosity of a sewage sludge, comprising:
(a) a device for concentrating and/or dewatering the sludge;
(b) a device in which the viscosity of the sludge obtained from the device of (a) is reduced;
(c) a device to subject the sludge obtained from the device of (b) to shearing; and
(d) means to control the flow and temperature of sludge, said means including means to subject the sludge to holding steps.
The term xe2x80x9csewage sludgexe2x80x9d as used herein may be broadly defined as xe2x80x9cwater-carried wastesxe2x80x9d, particularly from municipal sanitary sewage lines, containing one or more of body wastes, community waste such as street washings, etc., and industrial wastes and sludges. The total solids content of the sewage in municipal lines is usually about 0.5% w/w or more. The solids in sewage are mostly animal or vegetable substances, xe2x80x9corganic matterxe2x80x9d, i.e., fats, carbohydrates, and proteins. Some mineral or inorganic constituents present include sand and clay, as well as the common mineral salts found in the water supply.
Sewage sludge contains raw sewage and may contain a number of pathogens known to be health hazards to humans. Almost any type of microorganism may be found in sewage, including bacteria, protozoans, viruses and fungi. It is understood that raw sewage would normally have been passed through screens for the removal of large pieces of stone, wood, metal, and other trash which would clog channels or damage pumps. Coarse heavy inorganic materials, i.e. gravel, cinders, and sand are allowed to settle out. A sewage in the form of an aqueous suspension of sewage sludge and liquids is obtained. Concentration of the sewage sludge may be accomplished by any suitable way for separating solids and liquids, e.g. gravity settlings, filtration, centrifugation, hydrocyclone, or a combination thereof. Preferred methods of concentration include use of a screw press, belt press or centrifuge, or use of pressure, vacuum, heat or evaporation. It is the concentrated material that is preferably subjected to the processes of the present invention. The sludge that is treated according to the process of the present invention has a solids concentration of at least 10% (w/w).
In the concentration of the sewage sludge, a flocculating agent may be added to facilitate the concentration of the sewage sludge. A wide variety of flocculants may be used. The flocculants may be anionic or cationic polyelectrolytes, and in the form of polymers, copolymers and polymeric mixtures. Examples of the flocculants include polymers containing acrylamides, acrylate, quaternary acrylates; poly-N-vinylformamides, polyethyleneimine, chitosan, carboxymethyl-celluloses or -starches, cationized celluloses or starches and sodium alginate. For instance, the flocculating agent may be a cationic polyacrylamide copolymer, e.g. Percol(trademark) 757 cationic polyacrylamide copolymer. The flocculating agent, if used, is preferably used in concentrations of 0.01-0.05% w/v.
The pH of the concentrated sludge is adjusted by the addition of an alkaline material. The alkaline material may be any of a number of caustic compounds including mono- and di- valent hydroxides, especially alkali metal hydroxides. Examples include sodium and potassium hydroxide, soda ash, lime i.e. substantially pure calcium oxide, or other alkaline substances. The alkali may contain nitrogen. The selection may be dependent on availability and pH level desired e.g. sodium or potassium hydroxide may be required to obtain higher levels of pH.
The pH of the concentrated sludge is adjusted to the range of 9.5-12.5. In some jurisdictions, the pH of the sludge must be increased to at least 12.0 for a period of time. In such jurisdictions, compliance with regulations would be required. One such regulation is under the U.S. Environmental Protection Act, and requires municipalities to certify that treated sludge complies with all regulations. An example is the U.S. regulation of 40 C.F.R. Part 503.
It is preferred that the pH of the concentrated sludge be adjusted to the range of 10-12, especially 10.5-11.5. Thus, in the absence of regulations requiring use of a pH of at least 12, it is preferred to adjust the pH to the above ranges. If use of the higher pH is required, additional treatment at a pH of 10-12, especially 10.5-11.5, is preferred.
Subsequent to adjustment of the pH and prior to incubation, in one embodiment of the invention the solution of concentrated sludge is held for a period of time, especially held at ambient temperature and most particularly at 10-37xc2x0 C. for a period of time. The period of time may be several days, especially at least one day and preferably 3-10 days. The solution may be held for longer periods of time, but practical considerations in doing so may be important.
If the solution of concentrated sludge is held for a period of time prior to incubation, the subsequent step of addition of chemicals to further facilitate reduction in viscosity may be eliminated in one embodiment of the invention. Alternatively, the solution may be held for a period of time and then the step of addition of chemicals may be carried out.
In a preferred embodiment of the invention, chemicals are added to the concentrated sludge after adjustment of the pH as described above, to further effect a reduction in the viscosity of the sludge. The chemicals are selected to increase the ionic strength of the sludge, which may reduce the ionic interactions between filaments and/or particles and/or polymers in the sludge. Examples of the chemicals include salts of alkali and alkaline earth metals. Examples of cations include lithium, sodium, potassium, zinc, magnesium, calcium and manganese. Examples of anions include nitrate, sulphate and phosphate. The chemicals optionally may contain nitrogen. In embodiments, the salts are sodium or potassium halides, especially sodium or potassium chloride. Examples of the concentration range of the chemical additives are 0.1-1.0% w/v.
In addition, or alternatively, an oxidizing chemical may be added to the sludge. Examples of the oxidizing chemicals include oxygen, chlorine, perchlorate, perchlorites e.g. NaOCl, hydrogen peroxide, nitric acid, sulphuric acid, potassium permanganate, sodium perborate and ozone. Such chemicals may be used at concentrations of for example 0.001-0.01% w/v.
The sludge at the adjusted pH is subjected to an incubation step. It is understood that the incubation step follows at least one of the holding step and the chemical addition step. The incubation step is carried out at a temperature in the range of 40-100xc2x0 C. Preferably, the incubation step is carried out at a temperature of at least 50xc2x0 C., and especially 60-80xc2x0 C.
The incubation time will depend on the degree of reduction in viscosity to be attained and the temperature of the incubation step, and useful or optimal times may be determined. Longer times are required at lower temperatures. For instance, at 70xc2x0 C. in an embodiment of the invention illustrated herein, incubation times of 0.5-8.0 hours effected substantial reductions in viscosity, especially times of 1.0-2.0 hours. Periods of time substantially longer than optimal times may not be very beneficial.
The sludge is also subjected to a shearing step, a particle size reduction process to reduce particle size and/or break up agglomerates of particles in the sludge. For example, the sludge may be sheared at a temperature of from ambient temperature to 100xc2x0 C., and mostly preferably at about the temperature of the incubation step. The shearing step may be carried out simultaneously with and/or subsequent to the incubation step. In one embodiment, the shearing step is carried out simultaneously with the incubation step. In a preferred embodiment, the shearing step is carried out subsequent to the incubation step.
The shearing may take place in a high speed mixing apparatus equipped with one or more propellers, e.g. be effected mechanically by one or more high speed rotating blades. The blades may consist of turbine impellers or flat or toothed disks having high rotational speeds. Typical peripheral velocities of impellers used to achieve shearing are 1000-10,000 ft/min. or preferably 3000-6000 ft/min. Under these conditions, shearing may be carried out for 2 min to 2 hours. Other methods of shearing utilize high speed rotating blades within a cylindrical perforated grid which contributes to particle size reduction. Other systems reduce particles by application or release of pressure/vacuum, generation of cavitation forces in the sludge. Repeated freezing and thawing, passing sludge through small orifices causing abrasion, and passing frozen sludge under pressure through orifices where the ice crystals cause abrasion may be used
The method of the present invention may be operated as a continuous process, a batch process or combination of continuous and batch steps.
The present invention provides a method for reducing the viscosity of sludges having a solids content of  greater than 10% w/w. The treated sludges that are obtained are more readily pumped and/or transported, and thus more suitable for disposal.
The present invention also provides an apparatus for reducing the sludge viscosity of a sewage sludge. The apparatus has a device for concentrating and/or dewatering the sludge. Examples of such devices include screw presses, belt presses, centrifuges and filtration units. The apparatus also has a device e.g. one or more vessels to subject the sludge to holding and/or incubation steps, in order to reduce the viscosity of the sludge. In addition, the apparatus has a device to subject the sludge to a shearing step. Examples of such devices include a rotating toothed disc or impeller, especially with a tip speed of 1000-10,000 ft/min. Suitable controls to cause flow of sludge, including holding steps, are provided.